In this study, a small subfamily of double B-box zinc finger (DBB, DOUBLE B-BOX) genes, encoding eight putative transcription factors, were characterized with reference to the circadian rhythm and the early photomorphogenic regulation of hypocotyl elongation in response to light signals. Among these, it was found that the transcriptions of five DBB genes were under the control of circadian rhythm. To gain insight into the physiological roles of these putative transcription factors, forward and reverse genetic studies were carried out. The results suggested that they are commonly implicated in light signal transduction during early photomorphogenesis, however, their functions are not totally redundant, as judged by the fact that their circadian-expression profiles (or phases) were distinctive from each other, and by the fact that some DBBs (named DBB1a, DBB1b, STO, and STH) were apparently implicated in light signal transduction in a negative manner, whereas another (named DBB3) was implicated in a positive manner with regard to light-induced inhibition of elongation of hypocotyls.